Going back to copper litz fixed the problem ( and sounded better )
If you have the original wiring loom poor connections can be a source of rf - go through and clean all joints and resolder any dodgy looking soldered joints and check cartridge pin connections are nice and firm.
Failing that make sure that the wiring for each channel are a twisted pair - gently winding into a twisted pair +ve and -ve wires will help with the avoidance of rf.